• Object ID: 00000018WIA30945970GYZ
  • Topic ID: id_40025000 Version: 1.3
  • Date: May 12, 2022 11:38:40 PM

PROBE-S

PROBE-S is a version of the STEAM pulse sequence that acquires spectra from a single localized volume. The PROBE-S sequence is the automated PROBE/SVQ version of the STEAM CSI sequence with specialized data acquisition and data analysis capabilities. PROBE-S has no CSI data acquisition capabilities.

PROBE-S acquires echoes from a single localized volume defined by the intersection of three orthogonal slices. The PROBE Auto Prescan includes voxel localized shim, and AWS, an optional CHESS based, automatic water suppression process. The single voxel spectrum reconstruction includes a quantitative analysis of the spectrum that reports the signal intensities of choline, creatine, N-acetyl groups, myo-inositol, and the SNR of creatine. Water reference spectra acquired as part of the acquisition are employed in the reconstruction to determine and apply phase, residual eddy current and artifact corrections to the water suppressed data. The PROBE spectrum is drawn on a 512×512 image that is stored in the patient database. The results of the quantitative analysis are stored in the image header and are displayed with the Viewer.

PROBE-S is used to analyze the chemistry or levels of metabolites.

Consider this information when modifying PROBE-S scan parameters. For specific scan parameter values, select a protocol from your GE or Site library.

  • Scan selections: MRS Mode, Spectroscopy family, PROBE-S pulse.

General

  • If Auto Prescan is successful, the receive and transmit gains, the center frequency, the linewidth, the flip angle of the water suppression RF pulses, and the water suppression level are displayed. For example, Prescan values R1:11 R2:29 AX:63855957 LnWdth:5 Flip Ang:145 Supp Lvl: 98.
  • Auto Prescan can fail if the voxel volume is comprised of too varied anatomy. If Auto Prescan fails, it is most likely due to the placement of the voxel volume. Try repositioning the voxel volume to a more homogeneous anatomy and click Auto Prescan again.
  • The linewidth value is critical to the success of a spectroscopy scan. The shim process (part of APS) attempts to improve the homogeneity through the voxel by adjusting the gradient currents. The LnWdth value is a measure of the voxel homogeneity; the smaller the linewidth, the better the homogeneity. For single voxel spectroscopy, a linewidth value less than 0.1 ppm is acceptable. If the linewidth is greater than 0.1 ppm, recheck the position of the voxel and, if necessary, reposition the voxel to avoid magnetically inhomogeneous regions of the anatomy. Then click Auto Prescan again. When you are satisfied with the Auto Prescan results, click Scan. If the FWHM is still > 6 Hz, the scan may result in an unusable spectrum — the peaks are likely be broader than normal.
  • PROBE-P should be chosen preferentially before PROBE-S unless long-term, serial, or comparison studies with previous PROBE-S data is required or if specifically instructed by a radiologist or physicist. Otherwise, PROBE-P provides double the SNR as PROBE-S.
  • Each viewport of a single voxel VOI spectroscopy graphic prescription is screen saved and stored in a series numbered 5000+. Both the VOI and SAT pulses are saved with the images. When the spectroscopy acquisition is completed, the screen saved series is displayed in the Patient List and the three images can be viewed in the Viewer or Mini Viewer.
  • Phase cycling is the repetition of a pulse sequence and signal acquisition, such that all acquisition parameters except the phase of the RF pulses (and possibly the receiver) remain unchanged from repetition to repetition. Only the RF phase is changed, or cycled, as the repetitions occur. Phase cycling suppresses, or eliminates, undesirable signals while taking advantage of the effects of signal averaging.

Scan parameter

  • FOV: The minimum available FOV is displayed next to the text box and is the default FOV if the voxel is graphically prescribed without entering a value.
    • Voxel thickness is the depth of the FOV and the PRESS volume. Note that if you prescribe a small volume the SNR decreases proportionally.
  • NEX: The NEX value controls the number of RF phase cycles – the choices are 2 and 8. Set NEX to 8 to best reduce motion and susceptibility artifacts.
  • Scan plane: For graphic prescription of the PROBE volume, the plane of the localizer images must be the same as the prescribed plane. For example, an axial, PROBE ROI is prescribed on an axial localizer.
  • Slice thickness: While no CSI Slice Thickness value need be entered, set it to the voxel thickness value for consistency.
  • TE: Use the short TE (30 ms) to visualize short T2 chemical species such as glutamate, glutamine, and myo-inositol.
  • TR: TR must be > 1200 ms. The long minimum TR is required to accommodate the water suppression portion of the pulse sequence. Whatever your choice, it should be used routinely so that you can compare spectra acquired at different times, and in different patients. Most published spectra have been acquired at TE 1500, 2000, or 3000 ms. Ideally, the TR should be 3 to 5 times the longest T1 of the chemical species contributing to the spectrum.

User CVs

Click the Advance tab to view the available User CVs. The CVs may vary based on the field strength and selected scan and imaging parameters.